1. Field of Industrial Application
The present invention relates to a rotor for a dynamo electric machine in which materials for portions on the rotor such as wedges where an eddy current flows are improved.
2. Conventional Art
Iron or aluminum alloy is generally used for wedges which press field windings of a rotor for an electric generator used in an electric power generating plant using a gas turbine or a steam turbine as its prime mover.
In response to increasing electric power demands large capacity electric power generation plants are these days being constructed. In association therewith likely large starting devices are necessitated which are designed to start gas turbine or combined cycle power generation plants and a thyristor type starting system has been drawing an attention, in which during starting a power generation plant a synchronous generator in the power generation plant is operated as a variable speed motor and a gas turbine in the electric power genetation plant is accelerated upto a predetermined speed from where a self speed up is possible by a torque generated by the variable speed motor.
A rotating speed of a synchronous motor varies depending on frequencies of an AC power source applied to the armature windings thereof. Therefore, an AC power source of variable frequency is necessitated for operating a synchronous generator in variable speed, and the frequency thereof has to be adapted to be controlled dependent on the rotating speed of the variable speed motor. For the purpose of obtaining such AC power source of variable frequency a frequency converter device using semiconductor elements such as thyristors is made use of.
A frequency converter device is composed by a forward converting unit in which an AC is converted into a DC and a backward converting unit in which a DC is converted into an AC. When starting an electric power generation plant by making use of such frequency converter device, a rectangular shaped current is flowed through the armature windings of the electric generator. The rectangular shaped current contains higher harmonic current components as expressed by the following equations; EQU Frequencies of higher harmonic waves: fi=(6i.+-.1)f.sub.0 EQU Magnitudes of higher harmonic waves: Ai=A.sub.0 /(6i.+-.1)
wherein, i=1, 2, 3, . . . EQU f.sub.0 =frequency of fundamental wave EQU A.sub.0 =magnitude of fundamental wave
The higher harmonic current components flowing through the armature windings induce eddy currents on the surface of the rotor. Eddy current flowing through a wedge moves to teeth portions at a joint between adjacent wedges while bypassing the same. Further, eddy current moves to a retaining ring and a damper ring at the ends of the rotor and flows into the circumferential direction. Still further, at the magnetic poles eddy current flows in concentration near at the ends of cross slots because of restricted current flow passages. Due to these eddy currents flowing through the surfaces of the rotor resistance loss is generated to thereby raise the surface temperature of the rotor.
On one hand, a cooling performance of the rotor depends on its rpm, and when rotating in a low speed at the time of starting, the rotor shows an insufficient cooling performance to thereby raise a problem of increasing the rotor surface temperature during starting which causes an adverse effect to the electric generator. Further, since an electric power transmission system contains higher harmonic current components generated by many kinds of electric machines and apparatus included therein, an electric generator is always placed in a condition to be subjected to the higher harmonic current components.
In order to prevent an eddy current flowing through the surfaces of the rotor, it is conceived to concentrate the eddy current into wedges pressing the field windings in the rotor of the electric generator as well as into damper windings. However, when the wedges are made of one common material of iron, the resistivity thereof is substantially the same as that of the rotor such that it is difficult to concentrate the eddy current into the wedges. Likely, when the wedges are made of another common material of aluminum, the eddy current can be concentrated into the wedges, however such wedges raise another problem of poor resistance to electrolytic corrosion which is likely caused during such as at a low speed rotation.
JP-B-59-35420(1984) discloses wedges made of a Cu alloy containing Si and Ni of a few wt %, but pays no attention to electrolytic corrosion. JP-B-63-12931(1988) discloses a high mechanical strength and high electrical conductivity Cu alloy containing Si, Ni and Zr of a few wt % for electric machines, but again pays no attention to electrolytic corrosion as well as application to wedges and the like.
Further, it will cause a problem if a large current is flowed through the damper windings which are generally made of copper because of a relatively low heat resistance of the copper.